detection and discrimination of pure tones: characterised
by frequency and intensity

the complexity of spoken language, the use of telephones, the
effects of hearing loss

sound localisation (& the cocktail party effect) in auditory
scenes

can we generate auditory illusions ???

questions about hearing (auditory perception)

what is the significance of auditory perception for
a person in real life?

sounds signal events: therefore hearing provides an alarm system

emotional balance is influenced by sound (distress by noise, screeching,
relaxation by listening to music)

the auditory system is essential for communication - crucial for
human society

what are some of the most interesting problems in auditory
perception ?

the perceptual basis of harmony (why do some things sound pleasant?)

the influence of experience and knowledge on auditory perception

mechanisms of separating signal sources (localising the origin of
sound)

how does the brain allow for recognition of spoken words and voices

the nature of sound

a sound source is emitting circular pressure
waves (shells of air compression)

a sound source is emitting
circular pressure
waves (shells of air compression)

sound waves are similar to the radiating
ripples on the water surface, when a pebble is tossed into a still pond

a pure tone is
represented by a sinewave (air pressure as function of space/time)
which is travelling through space,
with amplitude and
frequency (1/period) corresponding to perceived loudness
and pitch

amplitude = distance between peak and trough of sinewave (overall size
pressure changes)
frequency = number of pressure changes per unit of time (inverse of sinewave
period, measured in Hz = cycles per second)

making music: the scale

notes of a musical score refer to the keys on the piano -> the frequency
generated -> the pitch of a musical tone

encoding of acoustic signals requires several 'engineering tasks', which are
accomplished by astonishing biological solutions:

outer ear: directional microphone

middle ear: impedance matching,
overload protection

inner ear: neural encoding, frequency
analysis

the sensory surface: inner ear

mechanical stimulation is transmitted through ossicles onto oval
window - here the osciallations are converted to pressure waves in
the cochlea - this generates
a travelling wave on the basilar membrane (which resonates
like the string of a guitar)

the organ of Corti in the cochlea picks up the vibrations from the basilar
membrane by means of hair cells: mechanosensor array

vary frequency of masking tone to determine how thresholds
change with frequency difference

systematic variation of frequency of masking tone to determine a set of detection
thresholds for a given target frequency (cf. Barlow & Mollon 1982) is a
key method to determine the auditory 'tuning curve'
for this target frequency: threshold amplitude as function of mask frequency
the 'bandwidth' of the frequency
filter that is detecting the target tone is given by the frequency differency
at halfheight of threshold function

tuning curve

low thresholds (mask amplitudes) for masks close to the target
frequency

high thresholds (mas amplitudes) for masks more distant from
target frequency

width of the U-shaped threshold curve corresponds to the bandwidth
of the frequency filter responsible for the target

this filter tuning is the basis for the perception
of pitch !

frequency tuning: basis of pitch perception

frequency tuning can be measured in systematic masking experiments for different
target tones …
many filters cover full range of frequencies – like digital audio systems!

electrophysiological measurements
from the auditory pathway (e.g. cochlear nerve of cat, sensitivity as
function of frequency) generate very similar patterns of frequency tuning
for individual neurones: preferred tones

many filters cover the whole audible frequency range

this filter
tuning is the basis for pitch discrimination involved in auditory perception,
such as recognising musical tunes!the same principle of encoding frequency components
separately is used in digital audio systems !!

how to measure loudness ...

the pressure of airwaves determines the magnitude of auditory sensation: 'loudness'

perceived loudness can be measured quantitatively
by comparing two successively presented tones (of different frequency)
and deciding which one sounded louder (forced choice FC to find threshold for
louder/softer)

intensity of comparison tone is adjusted until it has the same ‘subjective’
loudness as the reference tone
and then the physical intensity (sound pressure level SPL) is recorded as ‘perceived
loudness’

tinnitus: continuous humming or ringing, aftre some time it
leads to suppression of the affected frequencies

... imagine the consequences of such impairments of the audibility function
for communication and social life ...

auditory space

how does (perceptual) auditory space represent events in a four-dimensional
world (3 spatial dimensions + time)?
in vision, we have 2D-images, can easily localise objects in the visual field
and see several objects at the same time

but
can you hear things at the same time in different locations?

the ear is a 1D sensor (a microphone samples at one point in space) - so :

can we hear in two, or three dimensions?

can the auditory system localise objects?

can we hear several objects at the same time?

sound localization

there is no direct representation of auditory space :
location needs to be calculated from a number of cues (see Goldstein
2002, chapter 11)
(in biology there are experts for this job, like barn owls: see Konishi 1986)

pinnae : crucial for sensation
of space (reduced when using earphones!); used to locate elevation (up-down
dimension)

note the similarity between auditory stereo (this name
sounds familiar - your stereo system has two speakers!) and stereovision ....

the cocktail party effect

it is easy (for young folk) to single out one particular
voice from the background of a noisy pub,
or to pick up the tune of a single instrument from a large orchestra (this is
called the 'cocktail
party effect')

how can this be achieved?

the mixture of wavefronts hitting the ear has an overwhelming
complexity !

(more like the ‘rippled’ surface of a pond in a storm, from
Pierce 1983)

the core of the problem of the cocktail party effect (Cherry 1953) is masking
:
the detection of a tone is impaired if another tone or noise is presented at
the same time

masking depends on proximity
in space and similarity in frequency

binaural unmasking can be used
to separate sound sources in space: if spatial distance or difference in frequency
increases, separation becomes easier - these two cues are combined in the
binaural information (subtract signals from the two ears to unmask separate
sound sources)

high-level effects (attention,
familiarity of voice, language) & sensory fusion
(we use vision to support hearing) can also be used to separate sound sources
in space

auditory illusions

we can create illusions in the auditory system like in the visual system !